DR, E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
773 
The filaments appear to arise near the lips of the oesophagus, growing thence 
downwards along the septa. This suggests the possibility of ectodermic elements 
from the stomodseum entering into their composition, and I have made many 
longitudinal sections for the study of this point. Fig. 157 represents a longitudinal 
section through a larva of 100 hours (the mouth being fully formed) and the remains 
of the oesophageal plug (probably) being attached to a septum at pi. To the left is a 
mesenterial filament (f) clearly outlined and well differentiated from the rest of the 
septum by its more intense colour and granular appearance. Above, the ectoderm of 
the stomodseum may be very clearly distinguished from the entoderm by its less 
granular appearance and different colour. Following the ectoderm of the stomodseum 
downwards, it passes insensibly into the entoderm of the filament without any 
indication of a limit between them. On the right side, however, the ectoderm is 
separated below by a faint rounded outline, below which the entoderm is slightly 
thickened and more granular. The large granular mass is possibly a filament but 
more probably the oesophageal plug. The same general features are shown in fig. 158, 
and the ectoderm at the left side of the stomodseum becomes entirely continuous 
below with the entoderm of a mesenterial filament (/.). 
From these sections it might be concluded that the filaments are actually 
downgrowths from the stomodseum. In some cases, however, the filament appears to 
have at first no connexion with the oesophagus. This is shown for instance in fig. 159, 
where the filament (f) on the left side ends in front by a definite rounded outline and 
has no connexion with the oesophagus. It is possible that this thickening on the 
septum is not really a filament but a part of the oesophageal plug. Nearly conclusive 
evidence is however afforded by the section shown in fig. 137. In this specimen there 
is a thickening on the edge of a septum (f.) which is probably the beginning of a 
mesenterial filament before the cavity of the oesophagus has broken through, and there 
is no possibility of any communication with the stomodseal ectoderm. 
In Leptogorgia the filaments become visible shortly after the attachment of the 
larva. Two of them are much shorter than the others and are borne by a pair of 
septa which enclose one of the chambers at the angles of the elongated mouth ; they 
are in all probability homologous with the dorsal pair of filaments in Renilla. The 
six other filaments are much longer and are equal to one another in length. This 
arrangement was maintained almost unchanged for seven weeks when the young 
polyps were killed. 
Conclusions. 
The mesenterial filaments are at first purely entodermic structures, formed as 
thickenings on the edges of the septa. After the absorption of the bottom of the 
oesophagus the ectoderm of the stomodseum becomes directly continuous with the 
entoderm of the edges of the septa and mesenterial filaments. Hence the possibility 
MDCCCLXXXIII. 5 G 
